Development, replication, and reversion of mecillinam-induced ovoid form ofSalmonella typhimurium

Abstract

Concentrations of mecillinam ranging from 0.8 to 0.0125 μg/ml in Trypticase soy broth causedSalmonella typhimurium to assume an ovoid morphology. The presence of chloramphenicol or the absence of nutrients or nonphysiological temperatures prevented the change to ovoid morphology, whereas the presence of nalidixic acid modified the effect of mecillinam. Ovoid forms replicated as ovoids. No intermediate rod morphology was evident during eight daily subculturings of ovoids in mecillinam. Reversion of ovoids to normal morphology occurred within 3 h in drug-free medium, or within 15 min after the addition of penicillinase. Inhibition of either DNA or protein synthesis, or the absence of nutrients, or incubation in nonphysiological temperatures prevented reversion of ovoids to normal morphology.